This invention relates generally to the telecommunications arts and more particularly to line protectors for telecommunications circuits.
Generally speaking, a telephone line is made up of a pair of wires or conductors which are referred to as tip (T) and ring (R). In the telephone central office (C.O.), line protector devices are provided to protect the central office equipment from over-voltages and sometimes also from over-current which may occur on the incoming telephone line. The type of protection needed at the central office depends on the susceptibility of the incoming telephone lines to over-voltage conditions either alone or in combination with over-current conditions.
The incoming line pairs are connected to the T and R conductors of the C.O. equipment pairs at a connector block. Appropriate protector or protection devices are installed in the connector block so as to provide the desired over-voltage, and if desired also over-current protection for the incoming line pairs. Generally speaking, a separate protector device is used for each of the T and R conductors of each line pair. These protectors are interposed between each conductor of the pair and a ground circuit, such that upon the occurrence of an over-voltage condition of a specified magnitude, and, if desired, an over-current condition of a specified magnitude and/or duration, the incoming line will be momentarily or permanently shorted to ground. Generally speaking, a brief over-voltage condition will cause a temporary grounding of the affected line, until the condition has passed. On the other hand, an over-current condition of specified magnitude and/or duration often causes a permanent grounding of the incoming line which has experienced the over-current condition. In the latter case, it is necessary to remove and replace the protector device on the line which has experienced an over-current condition.
Prior art protector devices have utilized both a carbon-arc type and gas tube type over-voltage protectors. In each of these types of protectors, upon an occurrence of an over-voltage condition over or above a specified magnitude, an arc is passed across the device to cause a temporary grounding of the line on which the over-voltage condition has occurred. In order to protect against over-current conditions, the gas tube or carbon-arc arrester is normally combined with a heat-sensitive device. The latter heat-sensitive device is generally mechanically coupled and often spring-loaded within a connector block to short the incoming line to ground upon occurrence of a specified over-current condition which is sufficient to heat the heat-sensitive device beyond a predetermined threshold point. This threshold point is generally selected to correspond to the amount of heat produced by the over-current condition of specified duration. Usually, the heat-sensitive device is not a resetable or recoverable device and hence the protector unit must be replaced upon occurrence of an over-current condition sufficient to trip the heat-sensitive over-current protection device.
Other devices such as a vent-safe type gas tube or solid state over-voltage arrester have been utilized in the art. A vent-safe arrester generally provides a secondary or backup gap for over-voltage arcing to occur if for some reason the gas tube should vent and fail to arc. The secondary or backup gap assures that the over-voltage arrester will still conduct an over-voltage to ground at voltage level which provides adequate protection for the central office equipment.
One particularly well-known and successful type of prior connector block is known as the 300-type block and is often provided with protector units of the so-called 1A type. The 1A type of protector unit may provide over-voltage protection only or it may protect the associated conductor against both over-voltage and over-current conditions. While the 300-type block and 1A type protector unit have been widely accepted, there is nevertheless room for further improvement.